Archive for November, 2015

We’re increasingly familiar with the idea that in the marine world as the sea gets warmer, organisms will move north (or south) to higher latitudes to escape the growing heat. Much less familiar is the idea that in some places animals from very deep water will move into more shallow areas, and create havoc with the ecosystem existing there.

Enter King Crabs. These are large, sometimes huge, made at least a little famous by Alaskan King Crabs that starred in the reality show The Deadliest Catch. Around the globe, they tend to live in very cold, deep water, preying on bottom living mollusks, echinoderms and other crustaceans by crushing them.

Alaskan King Crabs are abundant on the sea floor of the Gulf of Alaska, the target of one of the world’s more risky fisheries (freerepublic.com)

A couple of other species live on the Continental Slope along the West Antarctic Peninsula, seaward of the more shallow Continental Shelf, mostly at depths between 2000 and 800 meters where water temperature varies from 0.4 degree C up to a balmy 1.16 degree C.

Like other King Crabs, they can tolerate very cold water, but there is a limit. They function well down to about 1 degree centigrade, but when the temperature is colder than about half a degree C, their magnesium physiology breaks down and they become paralysed and die.

Antarctic King Crab, Neolithodes yaldwini. The male is the larger one, guarding a female who he will mate with as soon as she molts (Katrien Heinman, nature.com)

Though that is certainly cold, sea temperatures on the adjacent more shallow Antarctic Continental Shelf remain even colder, a little below zero in all seasons, creating a lethal ceiling above the King Crabs deeper on the Slope.

This is all a relatively recent phenomenon. About 40 million years ago when the force of continental drift finally pushed Antarctica free from South America, the Antarctic Circumpolar Current formed, isolating the Southern Continent from the influence of more northerly warmer water, freezing the glaciers on the continental mass, and super-cooling the shallow seas on the Continental Shelf.

The Antarctic Circumpolar Current formed 40 million years ago, isolating and freezing Antarctica (globalspec.com)

For 40 million years, King Crabs have not been able to penetrate the colder shelf waters, and nor have predatory bony fish, sharks or rays, also unable to tolerate such cold temperatures. For 40 million years, the bottom living animals, mostly suspension feeders except for some predatory starfish and worms, have become lightly skeletized, soft, in the absence of the shell-crushing predators.

They remind us of the bottom living Paleozoic community last seen before fish (and King Crabs) evolved, 350 million years ago, a rare and possibly unique ecosystem today.

The animals living on the sea floor on the Antarctic Continental Shelf are unusual, soft-bodied echinoderms, mollusks and worms, susceptible to predation by shell-crushing King Crabs (nature.com)

But of course now things are changing. The Antarctic seas, especially around the West Antarctic Peninsula, are warming unusually quickly. Stronger winds, driven by climate warming, intensify the Antarctic Circumpolar Current, lifting warmer, denser, saltier water up from 4000 m over the lip of the Shelf and spilling into deeper canyons.

Marguerite Bay, West Antarctic Peninsula, where King Crabs have begun to appear in the deeper water canyons on the Shelf (pnas.com)

King Crabs are moving up into some of these canyons. The cold water ceiling above them on the Shelf is still there, but it is rising. Over the next decades – perhaps sooner than later – King Crabs will invade the rest of the Shelf.

The diverse deskeletized animals now living there will be then be history for they have no defenses against the King Crabs. The current fragile ecosystem will be disrupted, shifting toward something probably very similar to deep cold-water ecosystems elsewhere where King Crabs thrive.

At depths where King Crabs are common (black bars) potential prey (the other bar graphs) are greatly reduced (pnas.com)

This loss of an unusual ecosystem will be unfortunate, but there are obvious limits to our ability to be the stewards we might like to be. Clearly we have much more immediate and pressing problems to deal with. Still, we do know that there is so much that we but dimly understand about our current ecosystems, and as a result much of the change that lies ahead is simply unpredictable.

At least we can now add to the mix the idea that marine ecosystem change can come from any direction, including from below.